Japanese Patent No. 4067021 describes an electric motor controller. The electric motor controller has a converter and an inverter. The converter and the inverter are connected to each other through a DC link. The converter inputs thereto an AC (alternating current) voltage, performs a full-wave rectification thereon for conversion, and outputs a DC (direct current) voltage to the DC link. The inverter inputs thereto the DC voltage and performs conversion to output an AC voltage to an electric motor.
Further, the DC link is provided with an LC filter having a reactor and a capacitor. In more detail, the capacitor and the reactor are connected in series to each other between a pair of output ends of the converter, and a voltage across both ends of the capacitor is inputted to the inverter as the DC voltage. A capacitance of the capacitor is smaller than that of a so-called smoothing capacitor, and the voltage across both ends of the capacitor has a pulsating component caused by the full-wave rectification.
In Japanese Patent No. 4067021, in order to reduce a harmonic component of DC voltage caused by resonance of the LC filter, the inverter is controlled on the basis of a voltage across both ends of the reactor. For example, a correction of subtracting a value, obtained by multiplying the voltage of the reactor by a gain, from an initial value for a voltage control rate of the inverter is made to calculate a target value of a voltage control rate command. Then, a switching signal to the inverter is generated on the basis of the target value of the voltage control rate and a voltage command value calculated by a known method. This reduces the harmonic component of the voltage across both ends of the capacitor and further reduces distortion of the current to be inputted to the electric motor controller. The control based on a reactor voltage is referred to also as a “VL control system” in the present application.